ORGANIC LETTERS
Dimeric Assemblies from 1,2,3-Triazole-Appended Zn(II) Porphyrins with Control of NH-Tautomerism in 1,2,3-Triazole
2008 Vol. 10, No. 4 549-552
Chihiro Maeda, Shigeru Yamaguchi, Chusaku Ikeda, Hiroshi Shinokubo,* and Atsuhiro Osuka* Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan
[email protected];
[email protected] Received December 3, 2007
ABSTRACT
Cu(I)-catalyzed 1,3-dipolar cycloaddition of meso-ethynyl Zn(II) porphyrin with benzyl azide efficiently provides meso-1-benzyl-1H-1,2,3-triazolyl Zn(II) porphyrin, which assembles to form a slipped cofacial dimer by the complementary coordination of the triazole nitrogen atom at the 3-position to the zinc center of a second porphyrin moiety both in the solid and solution states. Removal of the benzyl protection and introduction of a 2-ethoxycarbonylphenyl moiety greatly stabilize the dimeric assembly through an additional hydrogen bonding interaction between the NH proton of 2H-1,2,3-triazole and the carbonyl oxygen.
Molecular fractionality given by tautomerism has been an intriguing topic in basic chemistry.1 In particular, (1) (a) Mo´, O.; De Paz, J. L. D.; Ya´n˜ez, M. J. Phys. Chem. 1986, 90, 5597. (b) Catala´n, J.; Claramunt, R. M.; Elguero, J.; Laynez, J.; Mene´ndez, M.; Anvia, F.; Quian, J. H.; Taagepera, M.; Taft, R. W. J. Am. Chem. Soc. 1988, 110, 4105. (c) Nagy, P. I.; Tejada, F. R.; Messer, W. S., Jr. J. Phys. Chem. B 2005, 109, 22588. (2) (a) Albert, R. A.; Taylor, P. J. J. Chem. Soc., Perkin Trans. 2 1989, 11, 1903. (b) Toma´s, F.; Abboud, J.-L. M.; Laynez, J.; Notario, R.; Santos, L.; Nilsson, S. O.; Catala´n, J.; Claramunt, R. M.; Elguero, J. J. Am. Chem. Soc. 1989, 111, 7348. (c) Toma´s, F.; Catalan, J.; Perez, P.; Elguero, J. J. Org. Chem. 1994, 59, 2799. (d) Abboud, J.-L. M.; Foces-Foces, C.; Notario, R.; Trifonov, R. E.; Volovodenko, A. P.; Ostrovskii, V. A.; Alkorta, I.; Elguero, J. Eur. J. Org. Chem. 2001, 3013. (3) Liang, Y.; Chang, C. K.; Peng, S.-M. J. Mol. Recognit. 1996, 9, 149. (4) (a) Brock, C. P.; Companion, A. L.; Kock, L. D.; Niedenzu, K. Inorg. Chem. 1991, 30, 784. (b) Li, D.; Li, R.; Qi, Z.; Feng, X.; Cai, J.; Shi, X. Inorg. Chem. Commun. 2001, 4, 483. (c) Yue, Y.-F.; Wang, B.-W.; Gao, E.-Q.; Fang, C.-J.; He, C.; Yan, C.-H. Chem. Commun. 2007, 20, 2034. 10.1021/ol7028299 CCC: $40.75 Published on Web 01/24/2008
© 2008 American Chemical Society
NH-tautomerism in 1,2,3-triazole between 1H- and 2H-forms has been argued on the basis of theoretical calculations, crystal structures, and basicity analysis (Scheme 1).2
Scheme 1. Tautomerism of 1,2,3-Triazole
Both forms are close in energy and easily interconvertible to each other in solution. Since 1,2,3-triazole plays a dual role as a donor and acceptor in hydrogen-bonding interactions, the fractional nature of 1,2,3-triazole would lead
to flexible molecular recognition.3 Furthermore, 1,2,3-triazole is also an important motif in coordination chemistry to construct metal complexes.4 Porphyrinic supramolecular assemblies constructed by noncovalent bonds have received much attention in wide areas of chemistry.5 Such noncovalent assemblies often employ coordination of pyridine and imidazole moieties to the zinc center of porphyrins.6 However, there are no precedent reports on the aggregation behavior of triazolylappended porphyrins. Recently, we have focused on 1,2,3triazolylporphyrins due to the synthetic ease of Huisgen’s 1,3-dipolar cycloaddition reaction between alkynylporphyrins and alkyl azides.7 Syntheses of triazolyl-appended porphyrins have been independently reported by the Chen and Odobel groups very recently.8 However, formation of molecular assemblies of 1,2,3-triazolylporphyrins both in solid and solution states remains unreported.
due to the aromatic ring current effect.9 The benzyl group was then removed with Pd/C and formic acid as the hydrogen source to provide 3 (Scheme 2). The 1H NMR spectrum of 3 in CDCl3 also displays typical features for dimeric porphyrin assemblies in a similar manner to 2 (Supporting Information, SI). Slow diffusion of acetonitrile into a chloroform solution of 2 afforded single crystals, suitable for X-ray crystallography. X-ray analysis of 2 unambiguously elucidated that 2 formed a dimeric assembly (2)2 constructed by the coordination between nitrogen and zinc atom (Figure 2a).10
Figure 1. 1H NMR spectrum of 2 in CDCl3.
A toluene solution of meso-ethynyl Zn(II) porphyrin 1 and benzyl azide was heated in the presence of a catalytic amount of CuCl. After chromatographic separation, meso-triazolyl Zn(II) porphyrin 2 was obtained in over 80% yield. Despite high temperatures, none of the other regioisomer was detected in the reaction mixture, probably because of the large steric effect of the porphyrin substituent. Importantly, the 1H NMR spectrum of 2 in CDCl3 exhibits a simple set of peaks with a large upfield shift for one β-proton at δ 5.7 ppm and for the triazole proton at δ 6.3 ppm (Figure 1). This characteristic is typical for dimeric porphyrin assemblies
Scheme 2. Synthesis of 1,2,3-Triazolylporphyrins 2 and 3
Figure 2. X-ray crystal structures of dimeric assembly of (a) (2)2, (b) (3)2, and (c) (7)2. Hydrogen atoms except NH protons and meso3,5-di(tert-butyl)phenyl groups were omitted for clarity. The thermal ellipsoids were at the 50% probability level. (d) N1-N2-N3 angle of triazole rings in 2, 3, and 7.
Furthermore, the slipped cofacial dimer structure of (3)2 was also revealed by X-ray analysis of 3 (Figure 2b).11 The coordination bond length in (3)2 is 2.13 Å and the distance between two porphyrin planes is 3.2 Å, which suggests π-π interaction. The Zn atom is displaced by 0.36 Å from the porphyrin plane. The structure of the triazole was determined (5) (a) Imamura, T.; Fukushima, K. Coord. Chem. ReV. 2000, 198, 133. (b) Wojaczynski, J.; Latos-Grazynski, L. Coord. Chem. ReV. 2000, 204, 113. (c) Ingeno, E.; Zangrando, E.; Alessio, E. Eur. J. Inorg. Chem. 2003, 2371. (d) Satake, A.; Kobuke, Y. Tetrahedron 2005, 61, 13. (e) Maeda, C.; Kamada, T.; Aratani, N.; Osuka, A. Coord. Chem. ReV. 2007, 251, 2743. 550
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Scheme 3. Synthesis of 1,2,3-Triazolylporphyrins 6 and 7a
to be the 1H-form on the basis of angles around the nitrogen atoms,2d indicating a preference for the 1H-tautomer in the solid state (Figure 2d). Although the 1H NMR spectrum in CDCl3 reveals complete formation of the dimeric assembly (3)2 at relatively high concentration (>1 mM), UV/vis absorption spectra in CHCl3 are concentration dependent in a range from 10-5 to (6) Hunter, C. A.; Hyde, R. K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1936. (b) Knapp, S.; Vasudevan, J.; Emeg, T. J.; Arion, B. H.; Potenza, J. A.; Schugar, H. J. Angew. Chem., Int. Ed. 1998, 37, 2368. (c) Wilson, G. S.; Anderson, H. L. Chem. Commun. 1999, 1539. (d) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538. (e) Haycock, R. A.; Yartsev, A.; Michelsen, U.; Sundstro¨m, V.; Hunter, C. A. Angew. Chem., Int. Ed. 2000, 39, 3616. (f) Ogawa, K.; Kobuke, Y. Angew. Chem., Int. Ed. 2000, 39, 4070. (g) Ikeda. C.; Nagahara, N.; Yoshioka, N.; Inoue, H. New J. Chem. 2000, 24, 897. (h) Ikeda, C.; Tanaka, Y.; Fijihara, T.; Ishii, Y.; Ushiyama. T.; Yamamoto, K.; Yoshioka, N.; Inoue, H. Inorg. Chem. 2001, 40, 3395. (i) Ogawa, K.; Zhang, T.; Yoshihara, K.; Kobuke, Y. J. Am. Chem. Soc. 2002, 124, 22. (j) Tsuda, A.; Sakamoto, S.; Yamaguchi, K.; Osuka, A. Angew. Chem., Int. Ed. 2002, 41, 2817. (k) Takahashi, R.; Kobuke, Y. J. Am. Chem. Soc. 2003, 125, 2372. (l) Tsuda, A.; Sakamoto, S.; Yamaguchi, K.; Aida, T. J. Am. Chem. Soc. 2003, 125, 15722. (m) Hwang, I.-W.; Kamada, T.; Ahn, T. K.; Ko, D. M.; Nakamura, T.; Tsuda, A.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2004, 126, 16187. (n) Kamada, T.; Aratani, N.; Ikeda, T.; Shibata, N.; Higuchi, Y.; Wakamiya, A.; Yamaguchi, S.; Kim, K. S.; Yoon, Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 7670. (o) Maeda, C.; Shinokubo, H.; Osuka, A. Org. Lett. 2007, 9, 2493. (7) Huisgen, R. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York, 1984. (8) (a) Shen, D.-M.; Liu, C.; Chen, Q.-Y. Eur. J. Org. Chem. 2007, 1419. (b) Severac, M.; Plex, L. L.; Scarpaci, A.; Blart, E.; Odobel, F. Tetrahedron Lett. 2007, 48, 6518. (9) (a) Kobuke, Y.; Miyaji, H. J. Am. Chem. Soc. 1994, 116, 4111. (b) Stibrany, R. T.; Vasudevan, J.; Knapp, S.; Potenza, J. A.; Emge, T.; Schugar, H. J. J. Am. Chem. Soc. 1996, 118, 3980. (10) Crystallographic data for 2: C142H164N14Cl18Zn2, MW 2907.82, triclinic, space group P-1 (No. 2), a ) 13.959(5) Å, b ) 14.301(5) Å, c ) 18.276(5) Å, R ) 95.686(5)°, β ) 99.037(5)°, γ ) 91.849(5)°, V ) 3581(2) Å3, T ) 90 K, Dcalcd ) 1.348 g cm-3, Z ) 1. For 18670 reflections measured; R1 ) 0.0991, wR2 ) 0.2463 for 12393 reflections with [I > 2σ(I)], GOF ) 1.117.
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10-3 M, showing the weak nature of this molecular assembly constructed by the coordination of 1,2,3-triazole to zinc atom. Upon an increase in concentration, the shape of the Soret band becomes dramatically broadened and Q-band becomes red-shifted (SI). A good fit for the observed sigmoidal curve was obtained by assuming porphyrin dimer formation, giving an association constant of K2 ) 1.0 × 104 M-1. This association constant, compared with that (K2 ) 1.0 × 1011 M-1) of a meso-imidazolyl Zn(II) porphyrin, indicates that the stability of (3)2 is only modest. A 2-ethoxycarbonylphenyl group was introduced at the meso-position opposite to the triazole group to explore extra stabilization of the dimer by a possible hydrogen bonding interaction.6h As shown in Scheme 3, 5-(2-ethoxyphenyl) substituted Zn(II) porphyrin 4 was prepared by SuzukiMiyaura coupling of 5-borylated Zn(II) porphyrin with ethyl 2-bromobenzoate. Subsequent bromination with NBS, Sonogashira coupling, and click reaction afforded 1,2,3-triazoleappended Zn(II) porphyrin 6. Finally, debenzylation was effected by Pd/charcoal to provide 7. Whereas the 1H NMR spectrum of 6 is simple in pyridine-d5, indicating its existence as a monomer, that of 6 in CDCl3 is slightly complicated, involving several upfield shifted signals. The latter spectrum indicated the presence of several not well-defined assemblies, which are probably constructed from atropisomers with respect to the direction of the ethoxycarbonyl group toward the triazole group. On the other hand, the 1H NMR spectrum (11) Crystallographic data for 3: C168H191N22Zn2, MW ) 2649.22, triclinic, space group P1, a ) 13.1202(13) Å, b ) 16.6253(16) Å, c ) 17.5706(17) Å, R ) 78.601(2)°, β ) 81.694(2)°, γ ) 89.735(2)°, V ) 3716.5(6) Å3, T ) 90 K, Dcalcd ) 1.184 g cm-3, Z ) 1. For 22936 reflections measured; R1 ) 0.0807, wR2 ) 0.1580 for 18998 reflections with [I > 2σ(I)], GOF ) 1.052. 551
of 7 in CDCl3 showed a simple set of peaks, indicating formation of a dimeric assembly of only one atropisomer. In the 1H NMR spectrum of 7, one of the pyrrolic β-protons was considerably shifted to high field at δ 5.7 ppm, and the triazole NH proton appeared at δ 8.7 ppm, suggesting hydrogen-bonding interaction. Finally, X-ray diffraction analysis clearly proved the dimeric structure of (7)2 (Figure 2c).12 The angle N1N2N3 was found to be 116°, which is typical for 2H-triazole.2d The distance between NH proton and carbonyl oxygen is 2.02 Å, supporting hydrogen bonding. Clearly, the hydrogen bond fixes the NH proton at the 2-position of 1,2,3-triazole. This finding is significant because it is usually difficult to determine the position NH proton of 1,2,3-triazole. The UV-absorption spectrum of 7 in CHCl3 shows splitting of the Soret band, which is characteristic for dimeric assemblies,9 down to 10-7 M (Figure 3). Fluores-
M-1 (SI).13 The additional two hydrogen bonding interactions, which fixes NH-tautomerism and atropisomerism at the same time, enhances the stability of the assembly by more than 6 orders of magnitude. In summary, meso-1,2,3-triazolyl Zn(II) porphyrins 2, 3, 6, and 7 were synthesized via click reaction and selfassembling behaviors to form dimeric assemblies by Zn-N coordination were confirmed by 1H NMR, X-ray analysis, and electronic spectra. Especially in the case of 7, the additional hydrogen-bonding provided by the remote 2-ethoxycarbonylphenyl group contributes to fixation of both NHtautomerism of 1,2,3-triazole and atropisomerization to afford a highly stable porphyrin assembly. Further investigations on the supramolecular chemistry and photochemistry of porphyrin arrays are currently being investigated in our laboratory. Acknowledgment. This work was supported by Grantin-Aids for Scientific Research (Nos. 18685013 and 18033028) and the Global COE Program “International Center for Integrated Research and Advanced Education in Materials Science, Kyoto University” (No. B-09) from MEXT, Japan. Note Added after ASAP Publication There was a spelling error in ref 8a in the version published ASAP January 24, 2008; the corrected version was published ASAP January 29, 2008.
Figure 3. UV-vis absorption spectra of (7)2 in CHCl3.
cence spectra are also concentration independent in the range from 10-8 to 10-6 M, indicating very tight assembly. In fact, the association constant is calculated to be K2 > 1.0 × 1011 (12) Crystallographic data for 7: C122H122Cl12N16O4Zn2, MW ) 2432.50, triclinic, space group P-1, a ) 17.527(5) Å, b ) 19.313(5) Å, c ) 19.410(5) Å, R ) 80.678(5)°, β ) 70.485(5)°, γ ) 70.749(5)°, V ) 5837(3) Å3, T ) 90 K, Dcalcd ) 1.384 g cm-3, Z ) 2. For 24729 reflections measured; R1 ) 0.0929, wR2 ) 0.2828 for 16916 reflections with [I > 2σ(I)], GOF ) 1.166.
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Supporting Information Available: Experimental procedures, compound data, association constants for 2, 3, and 7, and crystallographic data for 2, 3, and 7 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. OL7028299 (13) This strong binding is due to the cooperative effect of two Zn-N coordination bonds and two NH-O hydrogen bonds. This value is not too high according to the precedent reports.5d Unfortunately, the insolubility of 7 in methanol and DMSO did not allow us to investigate NMR spectra in these solvents. The 1H NMR spectrum of 7 in pyridine-d5 indicated complete dissociation.
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